A number of systems and techniques are employed to receive and measure various characteristics of a device under test (DUT), including periodically modulated output signals provided by the DUT in response to a stimulus signal. However, it is difficult to perform accurate and complete measurements of modulated output signals from a DUT using a conventional mixer-based receiver, where the difference between the minimum and maximum frequencies of the modulated output signal (referred to as “total bandwidth”) exceeds the intermediate frequency (IF) bandwidth of the receiver (referred to as “IF bandwidth”). The IF bandwidth of a receiver is the bandwidth over which a spectrum analyzer, for example, is able to can measure both amplitude and phase of the modulated output signal. For example, error-vector-magnitude (EVM) of a power amplifier (as the DUT) may be measured using a PNA-X network analyzer, available from Agilent Technologies, Inc., where the power amplifier is excited by a contiguously aggregated five-carrier LTE-A stimulus signal with a bandwidth of 100 MHz. Because of spectral regrowth, the total bandwidth of the amplifier output signal may easily exceed 300 MHz, whereas the IF bandwidth of the PNA-X receiver is only about 40 MHz.
Because of the limited IF bandwidth, at least eight local oscillator (LO) frequency settings are needed in the PNA-X receiver (for receiving the 300 MHz total bandwidth in eight 40 MHz bandwidth portions), along with eight corresponding analog-to-digital converter (ADC) data record acquisitions, in order to capture the spectrum between the lowest and highest frequencies of the total bandwidth (referred to as “full spectrum”). The full spectrum contains the signal power of the amplifier output signal. Each LO setting and corresponding ADC data record introduces an unknown phase shift characteristic to the corresponding measured spectrum. This unknown phase shift characteristic has both a constant and a linear component versus frequency. Because the aforementioned phase shifts are unknown, it is not possible to simply combine the eight measured spectra in the 40 MHz bandwidths into one 320 MHz wide full spectrum. Although the information of the eight acquisitions may be used to reconstruct amplitude of the full spectrum, allowing an accurate measurement of spectral regrowth parameters, such as adjacent-channel-power-ratio (ACPR), the phase information is distorted because of the above-mentioned unknown phase shifts, preventing determination of phase sensitive characteristics, such as EVM.
Accordingly, the current full spectrum measurement methodologies are insufficient for complete measurement of the full spectrum, and an accurate and efficient approach is needed.